Ah, no: incremental approaches only work in already well-formed code.

Poor code requires not coding but analysis and decisions, partitioning code and clients. So:

1. Stop writing code

2. Buy or write tools to analyze the code (modularity) and use-case (clients)

3. Make 3+ rough plans:

(a) leave it alone and manage quality;

(b) identify severable parts to fix and how (same clients);

(3) incrementally migrate (important) clients to something new

The key lesson is that incremental improvements are sinking money (and worse, time) into something that might need to go, without any real context for whether it's worth it.